A documentary critical of South Africa's president, Thabo Mbeki, is scheduled to be shown to the public tomorrow, more than a year after it was made and after twice being pulled from the state broadcaster amid accusations of political censorship.

The programme, which portrays Mr Mbeki as paranoid and vindictive, will be screened at an international film festival in Durban, coinciding with a conference of the ruling African National Congress overshadowed by a race for the party leadership.

Although Mr Mbeki opened the conference today with a denial that the leadership issue dominates the meeting, his centralising of power and controversial free market economic policies will undoubtedly overshadow the debates.

The documentary, commissioned and then canned by the South African Broadcasting Corporation (SABC) and seen by the Guardian, is critical of Mr Mbeki's style of leadership.

Allister Sparks, a former editor of the Rand Daily Mail who was interviewed for the programme, said he believes that the SABC management baulked at a sequence near the beginning of the film that shows Nelson Mandela speaking a decade ago as he handed the ANC leadership to Mr Mbeki who is sitting at his side.

Mr Mandela warns his successor against abusing power.

"There is a heavy responsibility for a leader elected unopposed. He may use that powerful position to settle scores with his detractors, to marginalise or get rid of them and surround themselves with yes-men and women," he said.

The documentary then goes on to build a picture of Mr Mbeki conducting himself in the manner Mr Mandela warned against. It describes how he centralised power and isolated himself from others in the party leadership.

The programme also reported on the purge of rivals, and the 2001 investigation of alleged plots to overthrow the president by some of the Mr Mbeki's principal ANC rivals, including the party's former general secretary, Cyril Ramaphosa, and Tokyo Sexwale, who is running to succeed the president.

Speaking at the ANC conference today, Mr Sexwale picked up on the theme. "There is a growing tendency to carry out dirty character assassinations and the dissemination of lies about other comrades. This has reached uncontrollable proportions," he said.

The SABC first declined to show the documentary a year ago and insisted on cuts, principally of a section that repeated unfounded rumours that Mr Mbeki was implicated in the murder by white rightwing extremists of the Communist party leader, Chris Hani, in 1993.

The revised programme was rescheduled for last month but then pulled again, because, the SABC said, "internal procedures were not followed".

Mr Mbeki's spokesman, Mukoni Ratshitanga, declined to comment on the content of the documentary but denied the presidency was involved in the decision not to show it. "It is absolutely the decision of the SABC to air or not to air the documentary and we have never interfered with it," he said.

Mr Sparks said the SABC told him it had declined to show the documentary because it was libellous. But he said he believes the broadcaster acted out of politically-motivated self-censorship because members of its board are mostly political appointments. "This leads to bias. There are members of the board with politically laden agendas," he said. "You can only conclude that this is a political decision. I think this was an act of self-censorship."